1. Field of the Invention
The present invention relates to improvements in an ozone generator and an ozone generating method and, more particularly, to improvements in an apparatus and a method of generating ozone by using silent discharge by application of a high-frequency electric field.
2. Description of the Related Art
In recent years, ozone is widely used in order to purify service water or highly treat sewage and liquid waste. In order to satisfy requests in these fields, a large capacity ozone generator capable of stably generating ozone is desired.
A conventional ozone generator generally comprises a discharge tube (dielectric high voltage electrode) constituted by a tube made of, e.g., glass and an aluminum film or the like formed on this tube. By performing discharge between the discharge tube and a ground electrode, oxygen in a feed gas is activated to produce ozone.
Upon production of ozone O.sub.3, a small amount of nitrogen oxide NO.sub.x is also produced. This nitrogen oxide reacts with moisture in the gas to produce nitric acid. This nitric acid corrodes the discharge tube, the ground electrode, and the like. In order to prevent the production of nitric acid, pure oxygen may be used as a feed gas. In this method, however, pure oxygen must be prepared. In addition, the moisture in the feed gas may be removed to supply a dry feed gas to an ozone generator. In this method, however, a dryer must be prepared independently of the ozone generator. And it is impossible to revome the noisture from the feed gas, and NO.sub.x is produced, though in very small quantities. NO.sub.x thus produced sticks onto the tubes. When the ozone generator is overhauled for inspection or repair, air flows thereinto, and NO.sub.x reacts with the moisture in the air, inevitably producing nitrogen acid.
In order to solve the above problems, a technique of forming the discharge tube, the ground electrode, and the like of the ozone generator by a material such as stainless steel having an ozone resistance and a nitric acid resistance is proposed in Published Examined Japanese Patent Application No. 57-47124. Stainless steel has good ozone and nitric acid resistances. For this reason, according to the technique described in this Published Examined Japanese Patent Application, the ozone generator can stably operate for a long time period even if nitric acid is produced together with ozone.
In order to improve an ozone production amount per unit volume of the ozone generator, a method of increasing the frequency of the power supplied to the discharge tube up to a high frequency from about 500 Hz to about 2,000 Hz (higher than a frequency of commercial power (generally 50 or 60 Hz)) is known. When the frequency of the power is increased to a high frequency, discharge electric power is increased to increase an ozone generation amount per unit volume of the ozone generator.
In an ozone generator including a discharge tube having a stainless steel film, however, if the generator is operated by setting a high frequency as a frequency of the power, a conductive film formed on a glass tube is sometimes peeled therefrom.